The present invention is related to excavating machines, and more particularly to excavating machines with improved motor control systems for controlling the drag and hoist drums.
A dragline is an earth working or excavating machine used in mining operations such as the extraction of coal, iron, copper or other minerals or materials. A typical dragline excavating machine includes a machinery house mounted on a platform supported for rotation. Extending from the machinery house is a boom supported by cables or lines, and held at a desired angle of inclination by pendants extending from the boom to a gantry mounted on top of the machinery house. A bucket is suspended from the boom by hoist ropes wound on hoist drums in the machinery house, and can be dragged toward the dragline excavating machine by coordinated motion of the hoist ropes and drag ropes. The drag ropes are wound on drums also housed in the machinery house. The machinery house includes drive systems for driving the hoist and drag motors, “swing” motors for rotating the machinery house, and, for moving or walking dragline excavating machines, drive systems for controlling the shoes and walking mechanism or for controlling a crawling device.
At excavation sites, alternating current (AC) utility power lines are typically provided to provide power for excavating equipment including the dragline excavating machines used at the site. The hoist and drag drums in the dragline, however, are very large, and draw a significant amount of power from the utility lines when in use. The drive systems for driving the hoist and drag drums, therefore, must be selected to provide sufficient power to drive the drums, and also must be selected to limit the effects on the AC utility power system, including harmonic distortion and power factor problems. Furthermore, to adequately provide excavation processes, it is important to be able to drive the drums at a very low speed.
Because of these problems, the drag and hoist drums of typical dragline excavators are operated by DC motors and associated motor-generator sets connected to the AC power line. The motor-generator sets each include a large synchronous AC motor driving DC generators, and are typically arranged in Ward-Leonard loop configurations in which the large synchronous motors are capable of controlling power factor to minimize power system effects.
While generally successful in powering dragline excavators with minimal effect on the power supply, there are a number of disadvantages associated with the motor-generator sets typically employed in these systems. First, because of the amount of force required to drive the drums, multiple drive motors must be provided for each drum. These motors require a significant amount of space in the machinery house, and further require a significant amount of maintenance.
Furthermore, to drive the drums at a sufficiently low speed, the DC drive motors are coupled to the drums through very large gear trains extending, in some cases, over 25 feet. These large gear trains also require a significant amount of space in the machinery housing, and further, are difficult to align accurately. The production and maintenance of such gear trains, is, therefore, both difficult and expensive, adding significantly to the cost and size of the resultant dragline excavator.
Because of these issues, since around 1980, more efficient AC drives have also been applied in mining excavator applications. These AC drives, however, typically use SCR rectifiers, and therefore suffer from high harmonic distortion and relatively low power factor. Because these devices have a significant detrimental effect on the AC utility power supply which can affect other devices using the utility power, AC drives have not been applied successfully to large dragline excavators.
There remains a need, therefore, for an improved system for controlling the drag and hoist drums in a dragline excavating machine, and particularly for an improved system which reduces the number of parts, decreases maintenance requirements, reduces the size of the equipment, provides increased machine productivity, reduces energy consumption, and simplifies manufacturing.